The invention relates to a superconducting device having a cryogenic system to whose cryogenic medium to be cooled by a cooling plant are thermally coupled                the superconductors of at least one superconducting unit and        the superconducting switching path, electrically connected to these superconductors, of a superconducting switch,the superconducting switching path being assigned a heater for a controlled transition of the superconducting material of the switching path into the normally conducting state. A corresponding superconducting device having such a cryogenic system is to be gathered from EP 0 074 030 A2.        
In the case of superconducting switches, the physical effect of the controllable transition from the superconducting into the normally conducting state can be used to implement a switching function, in particular in cases where no electrical isolation is required directly with the actual switching operation. Corresponding switches are used, in particular, in the field of NMR tomography (so-called “Magnetic Resonance Imaging [MRI]”) for medical diagnostics as so-called permanent current or short-circuit switches for superconducting magnets. In order to be able to apply current to the superconducting field windings of such units/magnets, the bridging superconducting switching path of the short-circuit switch must be opened. In this case, the superconducting state can be cancelled by raising the temperature above the critical transition temperature, increasing the electric current density and/or increasing the magnetic field at the switching path. Corresponding switching paths of superconducting switches that are to be controlled thermally have been known for some time (compare the EP-A2 document mentioned at the beginning, or from U.S. Pat. No. 3,255,335 or U.S. Pat. No. 4,602,231 A).
The superconducting switch is usually located inside a cryogenic system with a cryogenic medium that is also used for cooling the superconductors of a superconducting unit such as, for example, a field winding (compare the EP-A2 document mentioned at the beginning). As a result, when in the warm normally conducting state the switch introduces a substantial quantity of heat into the cryogenic medium of the cryogenic system. This quantity of heat can amount to a few watts in a liquid helium (LHe) bath of an MRI magnet. Such an input of heat is very often unacceptable. Thus, for example, for known MRI magnet systems where the aim is the complete recondensation of vaporizing helium by a cold head of a cooling plant, it is necessary for the power dissipated at a helium reservoir during ramping, that is to say heating of the superconducting switching path, to be limited, for example, to a few 0.1 W. However, this is inconsistent with the abovementioned losses in the switch. This problem therefore particularly affects recondensing, closed cryogenic systems where the cooling power of a cold head of a cooling plant is furnished in the form of a so-called cryogenic cooler. Such cryogenic coolers are, in particular, of the Gifford McMahon or Stirling type, or are designed as so-called pulse tube coolers.